1. Field of the Invention
This invention relates to a thin film transistor for use in driving an image sensor, an electroluminescence display, a liquid crystal display or the like; and more particularly to a method for fabricating a thin film transistor provided with a light intercepting layer for preventing the incidence of light on a semiconductor layer.
2. Related Art
FIGS. 7 and 8 show conventional thin film transistors in which the main part is comprised of a glass substrate (a), a gate electrode (b) formed on the glass substrate (a), a gate insulating film (c) for covering this gate electrode (b), and a semiconductor layer (d) deposited on the gate insulating film. A protecting film (e) is provided at a portion corresponding to the gate electrode (b) on this semiconductor layer (d) as necessary for protecting the semiconductor layer (d). Another semiconductor layer (f) is formed on the semiconductor layer (d) which is mixed with bivalent or trivalent atoms for forming an ohmic contact. A source electrode (g) and a drain electrode (h) are electrically connected to the semiconductor layer (f). A diffusion preventing layer (j) is disposed between the source and drain electrodes (g) and (h) and the semiconductor layer (f) for preventing the diffusion of metal composing the source and drain electrodes (g) and (h), as shown in FIGS. 7 and 8.
Further, a voltage (V.sub.D) is applied across the source and drain electrodes (g) and (h) and also a gate voltage (V.sub.g) is applied to the gate electrode (b). Thereby, a channel is formed in the semiconductor layer (d), and the transistor falls into an ON-state. In contrast, by reducing the gate voltage (V.sub.g), no channel is formed in the semiconductor layer (d) and the transistor falls into an OFF-state. Such a transistor is used for driving the above described image sensor, electroluminescence display, liquid crystal display, or the like.
In the thin film transistor of this type, a channel is formed in the semiconductor layer (d) even when light is incident on the semiconductor layer (d). Thus, such a transistor can fall into an ON-state in spite of the drop of the gate voltage (V.sub.g).
Therefore, the prior art employs a method for preventing the incidence of light on the semiconductor layer (d) by forming a light intercepting layer (k) made of a black resin (for example, a silicon resin containing carbon) on the top side of the thin film transistor.
There is, however, a problem in the prior art that the light intercepting action of the light intercepting layer (k) is not sufficiently effective so that where the thin film transistor is applied for driving of a device serving as an image sensor for example, the surface of the transistor can be irradiated by intensive light. Thus, part of the light can be incident on the semiconductor layer (d) whereby the transistor is constantly in an ON-state and thus the device does not function as an image sensor.
Further, there is another problem in the prior art that the number of steps for fabricating the conventional thin film transistor becomes large because of the fact that in fabricating the conventional thin film transistor, it is necessary to form the light intercepting layer (k) on the surfaces of the source and drain electrodes (g) and (h) after the electrodes are formed.